I am thinking that larger diameter rims/tires will lower engine RPM for a given vehicle speed. Am I correct here? Is there any info which provides this relationship?

 

Yes, increasing the over all tire size (either by increasing rim/tire size or just bigger tires) will reduce RPM's.
However, it will also reduce MPG's in stop and go traffic and may also place additional stress on automatic transmissions.

Now, this is assuming that you are increasing to a much larger over all size.
If your only going slightly larger, then you may not notice these things.

Also remember, even a small increase in over all tire size, you will affect the speedo and odometer readings.

Do a web search for "Plus Sizing" or "RPM Calculators".

Rich

 

What about the 12valves that have 4.10's? alot of people say the 12v engine and 4.10s is a "bad" combo. Could putting larger tires on these trucks actually improve performance? I have a stock (right now) truck, and am contemplating mild mods (nothing serious) and wondered if my gearing is actually hurting me. I am running 245/75/16's right now. Not trying to hijack this thread, just looking for input. -Tim

 

a larger diameter tire will "decrease" your effective gear ratio.... if you run a larger tire, your odometer will read less mileage, your speedometer will read less mph and you will run less rpm to go at a posted speed limit(because of the speedometer).... you will be able to gain some mpg but you will also have a larger rotating mass which will decrease your braking ability and may cost you slightly on your acceleration. It's a function of tire diameter.... if you run a 33" and your truck is calibrated for a 30", multiply your odometer and speedometer by the fraction 33/30 to attain the actual values... the same is true for any combination of tire diameters....